test: errorcheck auto-generated functions

test: errorcheck auto-generated functions

Add an "errorcheckwithauto" action which performs error check
including lines with auto-generated functions (excluded by
default). Comment "// ERRORAUTO" matches these lines.

Add testcase for CL 29570 (as an example).

Updates #16016, #17186.

Change-Id: Iaba3727336cd602f3dda6b9e5f97dafe0848e632
Reviewed-on: https://go-review.googlesource.com/29652
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>